In a data communication system, each communication terminals has a unique data communication address assigned thereto. For transmitting data from each communication terminal, a destination to which the data are to be transmitted is designated based on the data communication address. In a typical conventional data communication system, such a data communication address is fixed to each communication terminal and a network switching device (i.e., network exchange) determines the destination for the data based solely on the data communication address.
Incidentally, in the data communication system, sometimes a plurality of data terminal equipments are connected to one communication terminal. In this case, since the plurality of data communication terminals are used respectively by plurality of users, the data communication terminals are required to be handled independently. Further, the number of the data communication terminals, which are connected for use respectively to the communication terminal equipments and the data communication addresses for those data communication terminal equipments are changeable in accordance with necessity, taking into consideration convenience of the users of the respective terminals. However, since the conventional data transmission system is operated to control the destination for transmission of data by assigning a specific data communication address to each communication terminal as mentioned above, it apparently lacks versatility to meet such a request.